These routines, insert the complex character wch with ren-
dition before the character under the cursor. All charac-
ters to the right of the cursor are moved one space to the
right, with the possibility of the rightmost character on
the line being lost. The insertion operation does not
change the cursor position.

If successful, these functions return OK. If not, they
return ERR.
Functions with a "mv" prefix first perform a cursor move-
ment using wmove, and return an error if the position is
outside the window, or if the window pointer is null.